Finger-like macrovoid

Morphology of nanoparticles in the bulk of a polymer is shown in Figure 16.4 for WS2 (Lin et al. 2013). It can be seen that the nanoparticles are mainly present in between finger-like macrovoids and particularly on top of these macrovoids, in the thin separating layer of the membrane. 

In preparing membranes via the phase inversion process for applications in pressure-driven processes, the formation of macrovoids should be avoided completely. These finger-like pores of the type present in the substructure of membranes (b) and (c) of Fig. 3.6-1, severely Hmit the compaction resistance of the membrane. Membranes with a sponge-Hke structure (Fig. 3.6-la) are to be preferred. 

Weaker nonsolvents with a lower solubility parameter other than water, such as ethanol, are sometimes used. These weaker nonsolvents lead to the formation of a denser membrane (Albrecht et al. 2001 Young and Chen 1995). Systems with a rapid phase inversion rate (strong nonsolvent) tend to form macrovoids with finger-like structures, whereas systems with a slow phase inversion rate yield sponge-like structures (Young and Chen 1995). In addition, polymer crystallinity can be affected by the choice of nonsolvent, as reported by Buonomenna et al. (2007b) for the (DMA/ water and DMA/C1-C8 alcohols) system. 

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